1. Field of the Invention
This invention relates to a process for the purification of unrefined sugar solutions. More specifically, it relates to a process for removing turbidity, color, flavor, and odor from impure sugar solutions which may or may not be subjected to further crystallization.
Soft drink manufacturers virtually always require refined sugar for use in their beverages. However, many countries do not have sufficient refining capacity, and in these countries only mill sugar may be available to certain industries. Before using mill sugar for soft drinks it is necessary to remove from it the turbidity, color, flavor, and odor. The sugar syrups (or simple syrups) are generally made at about 60.degree. Brix.
The Cane Sugar Handbook, Meade and Chen, Tenth Edition, John Wiley and Sons, (New York, 1977) page 129, and references therein, each of which references is herein incorporated by reference, discloses in the manufacture of raw sugar (mill sugar), the dark colored raw cane juice, containing gums, waxes, proteins, organic acid, minerals, and particles of vegetable material, is first treated by adding lime to the hot juice. The lime reacts with the organic acids in the juice and forms an insoluble floc with various colloids and with the phosphates in the juice. The floc, containing impurities, is usually allowed to settle to the bottom of the reaction vessel. Alternatively, the floc may be removed by flotation. Polyelectrolytes are usually added to increase the size of the floc particles, and this "secondary" floc is then conventionally allowed to segregate or caused to rise by aeration, employing nozzle injection systems, high speed pumping or agitation. The floc-flotation-clarified cane juice is then evaporated in a multi-effect vacuum evaporator and crystallized in a vacuum pan.
The mixture of sugar crystals and sugar syrup or massecuite, is subsequently centrifuged to remove most of the dark mother liquor, or molasses, from the crystals. Residual molasses remaining on the crystals may then be removed with a water spray during continuing centrifugation. The greater the volume of wash water used to wash the crystals, the purer the resulting mill sugar will be. On the other hand, the more water used, the more the sugar crystals will dissolve, thus reducing the yield of mill sugar. However, no matter how thoroughly the crystals are washed, they may contain impurities occluded within the crystals. These impurities could be reduced by using more lime and phosphoric acid in the flocculation step, but again, at the expense of yield. Thus, even the best raw sugars (mill sugars) may contain various impurities.
Whereas, mill sugar is suitable in the preparation of products such as candy, bakery products, and sweeteners for coffee or tea, said sugar is undesirable in the production of soft drinks because the color, aroma, flavor, and turbidity of the sugar may affect the character of the soft drinks and shorten their shelf life.
Many sugar mills produce "sulfitated" sugars, wherein the cane juice is treated with sulfur dioxide prior to evaporation. These sugars often have a white appearance, which makes them suitable for certain uses, even though the aroma, flavor, and turbidity may not have been significantly reduced by this treatment. Mill sugar made with other special processing steps, such as extra washing, is called "mill white" or "plantation white," and is also suitable for certain uses. Generally, however, neither sulfitated sugar nor plantation white sugars are pure enough for use in soft drinks, in which higher quality refined sugars are necessary.
In manufacturing refined sugar, crystalline raw sugar, containing a number of undesirable non-sugar constituents, is first washed with water to remove any adhering syrup. The syrup that is washed off the crystals is similar in nature to raw cane juice and is treated separately to recover the sugar from it. The washed sugar is dissolved in water, and the resulting syrup is then clarified by floc-flotation after aerating by conventional means. The clarified syrup is then decolorized with activated carbon, bone char or other appropriate decolorizing substances to give a purified "fine liquor." The fine liquor is then crystallized to yield refined sugar. The degree of refinement depends on the number, and the effectiveness, of the flocculation and decolorization steps.
The flocculation steps using conventional techniques may comprise addition of lime and a phosphate ion source, such as phosphoric acid, to the liquor to form a calcium phosphate floc. The floc is conventionally removed by air flotation. A polyelectrolyte may be added in order to form a secondary floc, thereby increasing the size of the floc particles. Decolorization of the clarified liquor is usually accomplished by passing it through columns of bone char before the final crystallization.
The floc clarification process consists of adding to the dissolved sugar small amounts of lime and phosphoric acid, or lime and soluble phosphate salts or aluminum sulfate. At about neutral pH, the lime and phosphate or aluminum sulfate form an insoluble, primary floc wherein, said floc is calcium phosphate or aluminum hydroxide floc which contains insoluble matter, some of the colloids, and much of the color. The floc cannot be conveniently filtered because of its gelatinous nature. It will settle if given enough time, but it does not compact well enough to obtain a satisfactory yield of clarified syrup. Centrifuging in a continuous centrifuge is not satisfactory either, especially at higher sugar concentrations (50.degree. to 60.degree. Brix), probably because the turbulence in the centrifuge breaks the floc particles into smaller particles of a density about the same as or less than the syrup, so that a significant amount of floc is left in the syrup after centrifugation.
Another method for removing the primary calcium phosphate or aluminum hydroxide floc from sugar syrups by flotation with air. Generally, a polyelectrolyte is added to form a secondary floc to which gases will more easily adhere, thus making the flotation more efficient.
For flotation to occur, there must be enough bubbles which either adhere to or are captured by the secondary floc and rise to the surface of the syrup where a scum mat forms. The clarified sugar underneath is passed through a polishing filter and is then ready for use. The scum remaining in the tank contains sugar in an amount sufficient such that recovery of the scum is desirable. The recovery consists of mixing the scum with an appropriate amount of water, said amount of water being equal to or less than all the water required for the next batch. The resultant mixture is then aerated, more polyelectrolyte added, and flotation allowed to take place. A scum mat then forms on the surface. The clarified, dilute sugar solution is removed and may be employed in a new batch of higher Brix syrup. The scum containing some sugar may be discarded or recovered by a further recovery step or steps.
Mechanically generated air bubbles are commonly used in commercial floc flotation in sugar syrups, although carbon dioxide or oxygen bubbles are suitable for flotation of chemical flocs.
2. Discussion of Prior Art
U.S. Pat. No. 3,116,442 to Duke relates to the clarification of sugar liquor defecated with phosphoric acid and lime by mixing a small quantity of an organic amine, preferably a salt of a higher fatty amine, into the defecated liquor, incorporating a multiplicity of finely divided gas bubbles into the liquor containing amine at elevated temperature, maintaining the gas filled liquor quiescent at elevated temperature, and withdrawing from the top of the liquor a dark scum which is a concentrate of defecating agent with enmeshed impurities originally in the defecated liquor as well as color bodies originally in the defecated liquor, leaving a clarified and decolorized sugar liquor as a residue. U.S. Pat. No. 3,116,442 further discloses at Column 3, lines 27 to 30 use of carbon dioxide as the impregnating gas.
U.S. Pat. No. 3,479,221 to Buhl relates to a sugar purification process, wherein an aqueous sugar slurry containing sugar and impurities (either from cane or beet) is contacted with an acrylamide-beta methacrylyloxyethyltrimethylammonium methyl sulfate copolymer as an aid in flocculating and settling suspended solids. The use of said copolymer in the clarification step results in the copolymer flocculating the sugar impurities into large dense flocs which increases the settling rate of the impurities and the amount thereof settled.
U.S. Pat. No. 3,539,393 to Silva, et al. relates to a sugar clarification process comprising two-step heating with chemicals in which the sugar solution is initially treated with alum and pH stabilized with lime prior to the first heating and activated silica is added either before or after the second heating after which the solution is softened or deionized to remove scale forming minerals and/or other dissolved ions.
U.S. Pat. No. 3,853,616 to Rundell, et al. relates to a process for separating suspended solids from an aqueous sugar-containing liquor, which comprises forming a primary floc in the liquor containing suspended solids by phosphatation, preferably after treatment with a cationic surfactant, aerating the liquor containing the primary floc with agitation, distributing an organic polymeric flocculant uniformly throughout the liquid phase of the aerated liquor, to initiate the formation of a secondary floc therein, retaining the resultant mixture in a flocculator vessel with non-turbulent agitation preventing the segregation of the secondary floc to grow, transferring the liquor with minimal agitation, from the flocculator vessel to a separator vessel, allowing the secondary floc to segregate by flotation from the liquor in the separator vessel, and separately removing clarified liquor and flocculated solids from the separator vessel.
U.S. Pat. No. 3,926,662 to Rundell, et al. relates to a process for removing suspended solid impurities from sugar cane evaporated juice or sugar cane affination syrup, comprising adding a soluble phosphate sale to said sugar to form therein an insoluble calcium phosphate primary floc containing said suspended impurities, aerating the liquor containing the primary floc, with agitation, distributing uniformly throughout the aerated liquor from 1-40 parts by weight of an anionic flocculating agent per million parts by weight of sugar in the liquor to initiate the formation of a secondary floc therein, said flocculating agent being a polymer with a molecular weight from 1,000,000 to 10,000,000 containing 50 to 80 mol percent acrylamide units and from 50 to 20 mol percent of anionic units, retaining the resulting mixture for from 15 seconds to 5 minutes in a flocculator vessel with non-turbulent agitation preventing the segregation of the secondary floc from the liquor and allowing the secondary floc to grow, transferring the liquor containing the secondary floc with minimal agitation and shear from the flocculator vessel to a separator vessel, allowing the secondary floc to segregate by flotation from the liquor in the separator vessel, and separately removing clarified liquor and flocculated solids from the separator vessel.
U.S. Pat. No. 4,076,552 to Farag, et al. discloses a process for decolorizing sugar solutions with peroxide, which includes the steps of adding lime to the juice, adding peroxide, contacting the juice with carbon dioxide, filtering the juice, treating the juice with SO.sub.2, concentrating the juice and crystallizing sugar solids from the juice.
U.S. Pat. No. 4,196,017 to Melville, et al. relates to a method for reducing color impurities in sugar-containing syrups, comprising adding hydrogen peroxide to the syrup and mixing it with the syrup, adding a suitable cationic surfactant, and mixing, thereafter adding a suitable defectant, such as a mixture of calcium chloride and sodium carbonate, followed by adding and mixing activated vegetable carbon and diatomaceous earth, then filtering out the solids to obtain a purified sugar solution.
U.S. Pat. No. 3,909,287 to Rundell, et al. relates to a process for the recovery of sugar from clarifier scum by countercurrent extraction, wherein sugar is recovered from the clarifier scum by a multi-stage reflotation process involving at least two consecutive stages of counter-current aqueous extraction, using an organic polymeric flocculating agent.
A publication entitled "TALO Technology--a summary of processes, products and equipment from Tate & Lyle Engineering Ltd." discloses a process for scum desweetening which generally involves two or three consecutive stages of counter-current extraction with the desweetening water, wherein each extraction stage comprises the steps of:
(1) Dispersing the scum in desweetening water to give a homogeneous mixture and simultaneously aerating the mixture.
(2) Adding a solution of TALOFLOTE flocculant and dispersing this uniformly throughout the aerated mixture, to produce stable flocs which contain both scum particles and air bubbles.
(3) Passing the mixture from step 2. without any further agitation or shear into a special clarifier.
(4) Allowing the flocculated scum to separate from the mixture by flotation in the clarifier.
(5) Separately removing the clarifier sweetwater and flocculated scum from the clarifier.
U.S. Pat. No. 4,288,551 to Gudnason, et al. relates to a process for removing color, turbidity, flavor, and odor from impure sugar syrups by entrapping the sugar impurities in an insoluble, primary calcium phosphate or aluminum hydroxide floc at about neutral pH, adding a suitable amount of hydrogen peroxide with catalase to form a quantity of oxygen bubbles, and, during bubble formation, adding a polyelectrolyte to convert the primary floc into a secondary floc in which the oxygen bubbles are entrapped thereby causing flotation. The purified sugar syrup is then filtered with or without activated carbon and small amounts of a filter aid to produce a sugar syrup with substantially reduced color, turbidity, flavor, and odor.